Just a quick note. The GIMP “Auto” on the levels dialog is the same as running
Colors → Auto → White Balance
That is, on each RGB channel independently, discard pixels at each end of the histogram that are used by only 0.05% of the pixels in the image, and then stretch the remaining range as much as possible. This invariably leads to a shift in the hue of the image.
The thing is, curves can do all that levels can do, but much more. If you want to introduce folks to using them simply in a way that might mimic behavior of the Levels dialog, realize that pulling the white and black slider points in that dialog is the same as simply pulling the top and bottom of the curves around (and can be seen using the “Edit these Settings as Curves” option in GIMP).
(I would also strongly recommend to use a Logarithmic histogram in GIMP levels).
Is the same as:
The Output Levels can be controlled through the vertical axis in Curves.